Translocation and secretion

Growth at lower temperatures (e g, 30°C) may improve yield and activity of the secreted protein, possibly by allowing more time for membrane translocation and folding to occur (21) Variation of IPTG concentration and duration of induction may also affect the final yield It is beneficial therefore to cany out smaller scale expressions under various conditions to optimize these aspects... 

Intracellularly, S100A8/S100A9 complexes play an important role in myeloid maturation, cell trafficking and arachidonic acid (AA) metabolism. S100A8 and S100A9, present in high concentrations in the cytoplasm of neutrophils and monocytes, are able to translocate and are secreted after activation of a novel pathway requiring an intact microtubule network (Rammes et al., 1997). 

Thus, signal sequences are not gene-specific they can direct translocation of other secretory proteins. Furthermore, the secretion apparatus in one type of cell can recognize signal sequences from other types of cell, and secrete the attached protein. That prokaryotic proteins can be secreted in eukaryotic cells and vice versa indicates that the secretion apparatus has changed little (at least in some respects) in the process of evolution. 

Evidence for this model rests primarily on a correlation of lipid biosynthesis and translocation with protein synthesis and secretion. Increased synthesis of one causes increased synthesis of the other (Nesmayanova, 1982 Pag s, 1982). Both are inhibited by dissipation of the membrane potential (Bogdanov et al., 1984). Phosphatidylglycerol is present at the site of protein translocation, and may be involved in binding to the nascent chains and/or the ribosome (Bogdanov et al., 1985b). Bogdanov et al. (1985a) claim that secretion of alkaline phosphatase is accompanied by the appearance in freeze-fracture electron micro-... 

Thus, the temporal relationship between synthesis and secretion in E. coli remains somewhat unclear. Silhavy et al. (1983) and Rhoads et al. (1984) proposed that the coupling between the two processes is not as tight for prokaryotic secretion as it appears to be for eukaryotic secretion into the ER. The mode of secretion may be protein-specific. Some proteins [such as TEM j8-lactamase (Josefsson and Randall, 1981)], may be exported primarily posttranslationally, whereas some [such as PhoS (Pag s et al., 1984) and amp C /3-lactamase (Josefsson and Randall, 1981)] may be exported primarily cotranslationally in vivo. At least one protein that is secreted cotranslationally in vivo E. coli alkaline phosphatase) (Smith et al., 1977) can be translocated posttranslationally into E. coli membrane vesicles in vitro (Chen et al., 1985). The results of Ryan and Bassford (1985), described above, suggest that cotranslational export may be the major mode of secretion of wild-type proteins in normal (i.e., whole and not mutated) cells, while posttranslational secretion may be a backup system for use in case of damage to the secretory apparatus. 

The results described above in no way diminish the probability that various proteins are necessary for protein secretion, and that some of these proteins interact with signal sequences. Isolated signal sequences and precursor proteins have been shown to bind to proteins in the RER and to inhibit the translocation and processing of secretory proteins. 

Signal sequences are characterized by exceptionally diverse and numerous roles they are essential participants in the multistep process of protein secretion. While many details of this process are still poorly understood, it is clear that the involvement of the signal sequence includes recognition by proteins, interactions with membranes and/or membrane-resident components, facilitation of translocation, and specific cleavability. In these steps, the signal sequence probably undergoes conformational changes which themselves are required features of the mechanism. 

---